Performance Of Air-Entraining Agents At Low Temperature And Its Temperature Sensitivity Mechanism

The appropriate addition of air-entraining agent（AEA）can improve the workability of fresh concrete and the frost resistance of hardened concrete without significantly affecting the strength of concrete.The constructions in plateau show that low temperature will significantly affect the use of AEA.However,the research on performance change and related mechanism of AEA in concrete at low temperatures is insufficient.In this paper,the changes of air-entraining ability and retention rate of bubbles introduced at constant temperatures in different media,including solution,cement paste and concrete,were compared.And the mechanism of temperature sensitivity of saponin（SJ）,rosin（RO）and gemini air-entraining agent（MRP）was analyzed by comparing the effect of SJ,RO and MRP in solution system.The main results are as follows:（1）The constant temperature during the air-entraining test in different media was realized by the portable constant temperature and humidity chamber.The foam performance of SJ,RO and MRP solutions stirred at high speed at 0～30℃was obtained by using the blender test.Test results show that compared with results at 20～30℃,the effects of the low temperature at 0～15℃on the foamability and foam stability of the three AEA solutions are different,while the average bubble diameter increases with the decrease of temperature.（2）The variations of the surface activity,bulk viscosity and bubble membrane characteristics of the AEA solutions at 0～30℃were obtained.The grey correlation analysis results show that the most significant factors influencing the foamability of AEAs are the surface tensionγ,the maximum adsorption amountΓ_max and the minimum molecular area A_min.For the foam stability of AEAs,the key influencing factors areγ,Γ_max and the viscosityη.As temperature decreases,γ,Γ_max andηof the three AEA solutions all increase while A_min decreases.However,the change degrees of these factors are different,resulting a different trend of foamability and foam stability of the three AEA solutions with temperature change.（3）Based on the molecular structure analysis of AEAs,the mechanism of sensitivity of foamability and foam stability of AEAs was clarified.Due to the hydroxide radical capable of H-bonding with water in hydrophobic group of SJ,the hydrophobic effect of SJ is weaker than that of RO as the temperature decreases.The electrostatic repulsion of MRP is obviously larger than that of RO because of the two charged hydrophilic groups.The interaction force between the molecules adsorbed on the liquid film is different because of different molecular structures of AEAs,which influences the closeness of molecular arrangement.At low temperatures between 0℃and 15℃,the closeness order of molecular arrangement is RO>SJ>MRP,and this is an important reason affecting the order of foamability and foam stability of the three AEAs.（4）The air-entraining ability（initial air content and bubble diameter distribution）and bubble retention rate（air void content loss during curing process of cement paste and concrete,and 1 hour air content loss of fresh concrete）at 0～30℃were investigated.Test results show that the air-entraining ability of the three AEAs in cement paste weakens and the proportion of large bubbles increases as temperature decreases.In contrast,there is not obvious change trend of the air-entraining ability in the concrete system,thus effect of aggregate on the performance of AEAs is more obvious than temperature.The proportion of large bubbles in concrete increases as a whole at low temperatures between 0℃and 15℃.The retention rate of bubbles introduced by three kinds of AEAs increases in the two systems as temperature decreases.（5）The fractal dimension of air voids distribution（D_b）in hardened concrete at0～30℃was calculated.The results show that there is a significant positive correlation between D_b and bubble spacing factor in hardened concrete.The D_b of air entrained concrete decreases as the temperature decreases,indicating that the range of pore diameter is lower,the distribution of pore diameter is more concentrated and the frost resistance of concrete weakens.The hardened concrete mixed with MRP shows a larger D_b value at 0～15℃,and MRP concrete shows a better frost resistance at low temperatures compared with SJ and RO.